We survey the Si K edge structure in various absorbed Galactic low-mass X-ray binaries ( LMXBs ) to study states of silicon in the inter- and circum-stellar medium .
The bulk of these LMXBs lie toward the Galactic bulge region and all have column densities above 10 ^ { 22 } cm ^ { -2 } .
The observations were performed with the Chandra High Energy Transmission Grating Spectrometer .
The Si K edge in all sources appears at an energy value of 1844 \pm 0.001 eV .
The edge exhibits significant substructure which can be described by a near edge absorption feature at 1849 \pm 0.002 eV and a far edge absorption feature at 1865 \pm 0.002 eV .
Both of these absorption features appear variable with equivalent widths up to several mÅ .
We can describe the edge structure with several components : multiple edge functions , near edge absorption excesses from silicates in dust form , signatures from X-ray scattering optical depths , and a variable warm absorber from ionized atomic silicon .
The measured optical depths of the edges indicate much higher values than expected from atomic silicon cross sections and ISM abundances , and appear consistent with predictions from silicate X-ray absorption and scattering .
A comparison with models also indicates a preference for larger dust grain sizes .
In many cases we identify Si xiii resonance absorption and determine ionization parameters between log \xi = 1.8 and 2.8 and turbulent velocities between 300 and 1000 km s ^ { -1 } .
This places the warm absorber in close vicinity of the X-ray binaries .
In some data we observe a weak edge at 1.840 keV , potentially from a lesser contribution of neutral atomic silicon .